This invention relates generally to an ink jet printer of the type which provides ink on demand and more particularly to an ink jet printer where provision is made to eliminate gas from the ink supply and to overcome the effects of stagnating ink. The jet printer performs by ejecting ink droplets from a nozzle directly on to a recording medium such as paper. Many variations of ink jet printers have been suggested and some have been put into practice. As is well known, methods for printing by ink jet can be classified into two major groups. In a first method, ink is provided continuously as regular particles which are given an electrical charge. The charged particles are passed through an electrostatic field and deflected so as to form characters on the print medium. In the other method, ink is stored in an ink chamber having a side wall capable of deflection. In response to an electric pulse, the wall is abruptly deflected so as to reduce the internal volume of the ink chamber. The pressurized ink in the chamber is ejected from a small nozzle towards the recording medium. As a result, printing is accomplished on the medium in this on-demand type which is illustrated by U.S. Pat. No. 3,946,398.
Electrostatic deflection has several disadvantages, for example, the means for producing regular ink particles to be electrically charged is complicated. High voltage is needed for deflection of the charged particles and this equipment unfailingly uses more ink than is necessary to record the characters on the printed medium. As a result, the printing apparatus becomes large and complicated. Nevertheless, this method is nearing perfection and almost all printers currently in production are based on this method.
On the other hand, the method using the deflecting plate ejects ink in response to an electric pulse only as the ink is required. Accordingly, ink is never wasted in the printing process and printing is effective. Moreover, the voltage used for deflecting the wall is not high and as a result the apparatus is greatly simplified, small-sized, and a lower priced printer can be obtained. However, relatively few printers are produced using this method, and such printers have not been considered to be perfected.
An embodiment of a prior art printer using the deflecting wall method is shown in FIG. 1. A common ink chamber 2, pressure chamber 3 and nozzles 4 are formed as shallow depressions or grooves on a glass substrate 1 by etching or other means. Piezoelectric elements 5 are disposed in registry with the pressure chambers 3. By applying electrical pulses to the piezoelectric elements 5 with timing in accordance with printing signals, the piezoelectric elements 5 deflect inwardly to reduce the volume of the pressure chamber 3, thereby raising the hydraulic pressure of the liquid ink in the pressure chamber 3. As a result of the pressurization of the ink chamber, ink is ejected from the nozzle 4. Each nozzle 4 is capable of independent control by means of the individual piezoelectric elements 5.
Ink is ejected from a nozzle 4 only upon the application of any electric pulse to the associated piezoelectric element. Therefore, ink is never ejected without purpose in printing. The apparatus is much simpler than a printer using electrostatic deflection of ink particles.
However, some problems occur such as the irregular ejecting of ink or the clogging of ink in the vicinity of the nozzles 4. In view of the operating principles, it should be apparent that if the hydraulic pressure in the ink pressure chamber 3 is not sufficiently increased by the wall deflection, ink will not be uniformly ejected. This problem can arise when some gas bubbles, generally air, are mingled in the ink in the vicinity of the pressure chamber 3. When the pressure chamber wall is deflected, because the modulus of elasticity of a gas bubble is extremely less than that of the ink, the gas is compressed but the pressure of the fluid is not substantially increased. In other words, the gas is substantially more compressible than the liquid and the energy of deflection in the pressure chamber wall is absorbed in compressing the gas. Clogging is likely to occur because the diameter of the nozzle is extremely small and the hydraulic pressure at the time of ink ejection is less than that of the former method.
The problem of clogging has been resolved to a certain degree by using a cap over the tip of the nozzle. However, the cap mechanism are often complicated. Thus, in the prior art a perfect counter measure for the entrapment and mingling of bubbles has not yet been perfected. Therefore, the ink on-demand type printer has been delayed in achieving widespread use in practical applications.
With regard to the mingling of bubbles in the ink, there are several conditions which cause the formation or entrapment of bubbles. At the initial filling of the supply tank with ink, if there are some places where ink is apt to stagnate, the filling cannot be completed. Therefore, bubbles remain. There are also reasons attributable to the printer head construction. For example, a double cavity type printer head such as disclosed in U.S. Pat. No. 3,747,120, wherein the ink chamber is divided into two portions, has a disadvantage in that initial filling with ink cannot be perfectly completed. Consequently, in future printer head designs, the ink chamber should be constructed so as to be easily filled with ink when starting up or replenishing the ink supply. Also, there are conditions whereby bubbles are sucked in through the nozzles 4 during operation by getting a physical shock, or for other reasons. Bubbles can be absorbed by the evaporation of ink through the wall of the tube which connects the ink tank to the printer head and air can then permeate the tube from the outside into the ink supply. Also, when changing the ink cartridge, bubbles are frequently mixed at the point of connection.
For all these conditions, simple and logical means are needed for excluding bubbles if a printer of the ink on-demand type is to be put into widespread usage. In the prior art printers, some measures for the prevention of bubble mingling have been suggested, such as preventing an improper initial ink fill, preventing gas mingling when the ink cartridge is exchanged, and it has also been suggested to use degassed ink for filling the supply tank. However, at the conditions just described, it is nearly impossible to prevent some bubble intermingling.
The means for ink replenishment in the double cavity type of print head is extremely complicated (U.S. Pat. No. 4,015,272). Other devices provide a bubble trap along the length of the ink delivery tube, however, the mechanism becomes large and complicated and has little effect against minute bubbles. Another printer provides a means to exclude bubbles by applying pressure on the ink cartridge and ejecting gas bubbles through the nozzles along with ink. However, this is not a suitable approach because much ink is wasted (U.S. Pat. Nos. 4,123,761 and 4,074,284).
In another example, an exhaust mechanism is provided in the head (U.S. Pat. No. 4,126,868), however, ink which stagnates in an upper portion can be sucked into the nozzle, for example, if the head receives a physical shock. Minute bubbles attached to walls of the exhaust tube are difficult to bring to the upper portion, so that much ink is wasted when bubbles are excluded by flowing of ink.
What is needed is an ink jet printer having simple means for excluding bubbles effectively from the ink supply and also to dissolve or remove clogged ink.